Add Copilot instructions for repository context

.github/copilot-instructions.md

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+# Copilot Instructions for GitInsights
+
+## Repository Overview
+
+GitInsights is a Go-based command-line tool that analyzes GitHub profiles and generates detailed statistics about repository languages, commit patterns, and productivity metrics. The tool updates a README.md file with visual insights about a user's GitHub activity.
+
+## Technology Stack
+
+- **Language**: Go 1.21
+- **Key Dependencies**:
+  - `github.com/google/go-github/v38` - GitHub API client
+  - `golang.org/x/oauth2` - OAuth2 authentication
+- **Testing**: Standard Go testing framework
+- **CI/CD**: GitHub Actions
+
+## Project Structure
+
+```
+.
+├── main.go           # Main application logic
+├── main_test.go      # Unit tests with mocked GitHub API
+├── go.mod            # Go module dependencies
+├── go.sum            # Dependency checksums
+├── README.md         # Project documentation with embedded statistics
+└── .github/
+    └── workflows/    # GitHub Actions workflows
+```
+
+## Key Components
+
+### Core Functionality
+
+1. **GitHub API Integration** (`GitHubClient` interface):
+   - `GetUser()` - Retrieve user details
+   - `ListRepositories()` - Fetch user's repositories
+   - `ListLanguages()` - Get language statistics per repository
+   - `ListAllCommits()` - Retrieve commit history across repositories
+
+2. **Statistics Generation**:
+   - `summarizeGitHubProfile()` - Aggregates language usage across all repositories
+   - `calculateMostProductiveDay()` - Analyzes commit patterns by day of week
+   - `calculateMostProductiveTime()` - Analyzes commit patterns by hour
+   - `generateMarkdown()` - Creates formatted output with progress bars
+
+3. **README Update**:
+   - `updateReadme()` - Replaces content between `<!--START_SECTION:GitInsights-->` and `<!--END_SECTION:GitInsights-->` markers
+
+## Development Workflow
+
+### Building
+
+```bash
+go build ./...
+```
+
+### Running Tests
+
+```bash
+go test -v ./...
+```
+
+### Linting
+
+```bash
+go vet ./...
+gofmt -l .
+```
+
+### Running the Application
+
+Requires `GITHUB_TOKEN` environment variable:
+
+```bash
+export GITHUB_TOKEN=$(gh auth token)
+go run main.go
+```
+
+## Code Style and Conventions
+
+1. **Interface-Based Design**: Use the `GitHubClient` interface for GitHub operations to enable mocking in tests
+2. **Concurrency**: Use goroutines and channels for parallel API calls (see repository and commit processing)
+3. **Error Handling**: Always return and check errors; log non-fatal errors with `log.Printf()`
+4. **Testing**: Mock the `GitHubClient` interface using `MockGitHubAPI` for unit tests
+5. **Formatting**: Code must pass `gofmt` checks (enforced in CI)
+
+## Testing Guidelines
+
+- All tests use mock implementations of `GitHubClient` interface
+- Mock data should be realistic and cover edge cases
+- Test functions follow the pattern `Test<FunctionName>`
+- Use the `MockGitHubAPI` struct defined in `main_test.go`
+- Tests should verify both success cases and error handling
+
+## Important Notes
+
+1. **README Markers**: The application expects `<!--START_SECTION:GitInsights-->` and `<!--END_SECTION:GitInsights-->` markers in README.md
+2. **Language Aggregation**: Languages below 5% threshold are combined into "Other" category
+3. **Concurrency**: Repository and commit processing uses goroutines for performance
+4. **Authentication**: Requires valid GitHub personal access token with appropriate scopes
+
+## GitHub Actions
+
+Three workflows are configured:
+
+1. **test.yaml** - Runs on push/PR: tests, linting, formatting checks, build
+2. **build_and_release.yaml** - Creates releases with compiled binaries
+3. **update_readme.yaml** - Scheduled/manual execution to update README with latest statistics
+
+## Making Changes
+
+When modifying code:
+
+1. Maintain the interface-based architecture for testability
+2. Add corresponding unit tests with mocked dependencies
+3. Ensure concurrent operations use proper synchronization (WaitGroups, channels)
+4. Run full test suite before committing: `go test -v ./...`
+5. Verify formatting: `gofmt -l .` (should return no files)
+6. Run linter: `go vet ./...`
+7. Test builds successfully: `go build ./...`
+
+## Common Tasks
+
+### Adding New Statistics
+
+1. Create a new function following the pattern of `calculateMostProductiveDay()`
+2. Add a method to `GitHubClient` interface if new API calls are needed
+3. Implement the method in both `GitHubAPI` and `MockGitHubAPI`
+4. Update `generateMarkdown()` to include the new statistic
+5. Add comprehensive unit tests
+
+### Modifying Output Format
+
+1. Update `generateMarkdown()` function
+2. Maintain the `<!--START_SECTION:GitInsights-->` and `<!--END_SECTION:GitInsights-->` markers
+3. Test with `updateReadme()` function to ensure proper section replacement
+
+### Adding API Endpoints
+
+1. Add method to `GitHubClient` interface
+2. Implement in `GitHubAPI` struct using `g.Client` (go-github client)
+3. Add mock implementation in `MockGitHubAPI` for testing
+4. Write unit tests using the mock

.gitignore

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+GitInsights
+*.out